1. Field of the Invention
The present invention relates to an optical scanning device and, more particularly, to an optical scanning device suitable for an apparatus such as a laser beam printer (LBP) or the like, which records an image by optically scanning a photosensitive body surface as the surface to be scanned with a plurality of light beams (bundles of rays), which are emitted by a plurality of light-emitting portions of a light source means, and are guided toward the photosensitive body surface via a light deflector such as a rotary polygonal mirror or the like.
2. Related Background Art
As a conventional optical scanning device for attaining high-speed printing, a multi-beam optical scanning device has been proposed and manufactured. Such multi-beam optical scanning device has a plurality of light-emitting portions (laser tips) as a light source means, and simultaneously optically scans a recording medium surface with a plurality of beam spots which are formed on the recording medium surface by imaging a plurality of light beams emitted by the plurality of light-emitting portions by an imaging lens via a light deflector.
In order to attain high-speed printing using only one beam spot (light beam), the number of times of optically scanning the recording medium surface per unit time must be very large, and as a consequence, the rotational speed of the light deflector, image clocks, and the like cannot follow such large number of times of optical scanning. Accordingly, if the number of beam spots that simultaneously scan the recording medium surface is increased, the rotational speed of the light deflector, image clocks, and the like decrease in inverse proportion to the number of beam spots.
As the most efficient method of forming a plurality of beam spots, a laser element serving as a light source has a plurality of light-emitting points (light-emitting portions) which can be driven independently. Such laser element having a plurality of light-emitting point s is generally called a "monolithic multi-beam laser element". When the monolithic multi-beam laser element is used, various optical elements arranged after the light source can be commonly used by the plurality of light beams, thus providing large merits in terms of cost, working, adjustments, and the like.
The monolithic multi-beam laser elements include, e.g., a so-called surface-emission laser (surface-emission type semiconductor laser).
The surface-emission laser emits a light beam in a direction parallel to the direction of thickness of a silicon layer, while a conventional semiconductor laser emits a light beam in a direction perpendicular to the direction of thickness of the silicon layer, and the surface-emission laser has the following feature. That is, the conventional semiconductor laser emits divergent light, which has an elliptic section and suffers large variations in divergent angle, while the surface-emission laser can emit a circular beam having a stable divergent angle.
However, the surface-emission laser has a problem, i.e., has an unstable direction of polarization of the output light beam. Although the direction of polarization can be controlled to some extent by the manufacturing method, the direction of polarization fluctuates depending on variations in units of light-emitting points, ambient temperature, and outputs.
Normally, the reflectances, transmittances, and angle characteristics of optical elements that build the optical scanning device, e.g., a polygonal mirror as a light deflector, a scanning lens (f-.theta. lens) as an imaging optical system, a return mirror for changing the optical path, and the like change depending on the direction of polarization of the input light beam.
For this reason, when a monolithic multi-beam laser element comprising a surface-emission laser is used as the light source of the optical scanning device, a plurality of beam spots that optically scan the recording medium surface have different intensities in correspondence with the different directions of polarization of the individual light-emitting points, and such differences in intensity appear as pitch nonuniformities on an image, thus considerably deteriorating the image quality.